Recent studies worldwide indicate that while some systems and mobile operators are in desperate need for more efficient utilization of spectrum resources, most of the radio spectrum resources remain underutilized or unused most of the time. The increasing demand for flexible use of the radio spectrum for emerging new services and applications is the motivation behind numerous research activities worldwide. Efficient access to radio spectrum resources will generate new sources of revenues for worldwide vendors and wireless network operators. The new design of wireless radio infrastructure outlines the new attempts to share the spectrum in a fundamentally new fashion which would ultimately lead to better utilization of spectrum. The proposed spectrum management architectures and the spectrum sharing functionalities developed recently will result in reducing the time required to tailor a new service to an operator network. Furthermore, the flexible spectrum access and usage leads to more capable and faster services with high Quality of Service (QoS), giving more user satisfaction than conventional networks.
Four different levels of spectrum management techniques have been introduced in three different time scales: Spectrum Sharing and Coexistence (couple of hours or days), Long Term Spectrum Assignment (couple of minutes), Short Term Spectrum Assignment (1 sec or tens of ms) and fast dynamic spectrum allocation (10 ms time scale or even below). One of the elements included in this system is the concept of a gateway, which provides for LT spectrum assignment and some radio resource management (RRM) algorithms. One of the most challenging scenarios for spectrum sharing is short-term inter-mode spectrum assignment in a hierarchical overlaid cellular network. Unlike the intra-RAN spectrum sharing and exchange, where adjacent cells are exchanging spectrum, in a hierarchical overlaid cellular network, where the cells are no longer adjacent and are overlapped, the short-term and long-term spectrum assignment are challenging tasks. When, for example, Metropolitan Area (MA) cells with Time Division Duplex (TDD) operations are located within a Wide Area (WA) cell with Frequency Division Duplex (FDD) operations, and an MA cell or base station is to assign a chunk of spectrum to a WA cell, since all the MA cells are surrounded by WA cells, any allocation of spectrum to WA cells would ultimately lead to severe interference being inflicted on other MA cells. FIG. 1 shows a hierarchical overlaid cellular network having three WA cells WA1-3 having respective base stations BS1-3 each operating using an FDD mode. MA cells MA1-3 of a first MAN are located within cell WA1, MA cells MA4-6 of a second MAN are located within WA cell WA2, and MA cells MA7-9 of a third MAN are located within WA cell WA3. Since each MA cell uses a TDD mode, and the three MANs are geographically separated, each MA cell is able to operate within the same spectrum band.
There may be a situation in which one of the cells (say for example MA1 in FIG. 1) has spectrum resources available for sharing and allocation to WA1. Referring to FIG. 2, if the spectrum assignment (indicated by solid arrows) happens without coordination with other involved MA cells, the result is severe interference from the WA cell on the cells MA2 and MA3. The dotted lines in FIG. 2 indicate the interference and the portions of spectrum in which it may occur. As shown in FIG. 2, use by the cell WA1 of the chunks of spectrum assigned from MA1 to WA1 has caused interference on MA2 and MA3 as the latter cells are still operating in TDD mode within the chucks of spectrum given away by MA1. Unlike adjacent cells, which can be separated from each other by employing a safe separation distance or an exclusion zone, for the hierarchical overlaid cellular network, the proposed safe separation distances or exclusion zones are difficult to realize.
FIG. 3 shows a situation in which the assignment of spectrum to WA1 has resulted in interference (indicated again by dotted arrows) on MA cells within other MANs located in other WA cells. If the MA cells in the other WA cells are located far enough from WA1, the interference may not be critical.